Coherent recombination of ultra-short pulse laser sources applies notably to the realization of high-energy laser sources.
Methods for spatially recombining coherent beams fall into 2 categories, depending on whether one chooses to juxtapose the optical beams in the far field or to superpose them in the near field, that is to say at the level of the exit pupil of the system.
A system for recombining by juxtaposition is shown in FIG. 1a. In this case, the beams to be recombined, arising from laser sources Fk, k varying from 0 to N, are parallel and collimated in the near field by an array of collimating lenses MLC, and are disposed alongside one another, in the most compact manner possible. The superposition of the beams is then performed by free propagation up to the far field. Such a system does not involve any dispersive hardware components and therefore applies equally for pulse widths of less than a picosecond. However, the major drawback of this system is its relatively low efficiency, with notably an appreciable share of the energy lost in the grating lobes.
In the case of a near-field superposition system, it is for example possible to recombine the optical beams by using the polarization of the electromagnetic field: the optical beams arising from the laser sources Fk and collimated by collimating lenses CLk are superposed in the near field by means of polarization-splitter cubes PBSk respectively associated with half-wave plates HWPk, as illustrated by the example of FIG. 1b. According to this system the recombining efficiency for N beams is given by:
  Eff  =            1      N        ⁢          (                        η                      N            -            1                          +                              ∑                          k              =              1                                      N              -              1                                ⁢                                          ⁢                      η            k                              )      where η is the coefficient of transmission of each pair (polarization-splitter cube/half-wave plate). The advantage of this architecture is its relative simplicity of implementation for a reduced number of beams to be recombined: typically about ten at the maximum. For a large number of beams, on the one hand the implementation of the system becomes very complex, and on the other hand, the recombining efficiency drops rapidly with the number of sources (for η=99%, the efficiency drops to 10% for 1000 recombined beams).
Whether involving recombination in the far field by free propagation of collimated and parallel beams, or superposition of the near-field beams by using a splitter plate or a polarization-splitter cube, none of these systems is suitable for recombining a large number of pulses (typically >100 or indeed 1000), i.e. due to problems of efficiency (grating lobes for the far-field device), or of implementation for near-field systems.
Another technique for recombining by superposition uses a diffractive optical element to combine the beams. According to this technique illustrated in FIG. 1c, a lens 23 in a Fourier-transform setup makes it possible to collimate the beams to be recombined (arising from the laser sources Fk) and to direct them toward a diffractive optical element or DOE 1 situated in the focal plane of the lens 23. The spatial distribution of the source points in the object plane A of the lens 23 (periodic distribution of period PA) is transformed into a distribution of angles of incidence on the optical element DOE 1. The optical element 1 is typically a periodic phase grating, for example of Damann grating type, which ensures the constructive interference of all the incident beams on the order 0, and destructive on all the other orders; the period Λ of this grating and the angles of incidence θ2k are related by the known formula for diffraction gratings:
      sin    ⁡          (              θ                  2          ⁢                                          ⁢          k                    )        =      k    ×                  λ        0            Λ      
The advantages of this architecture are notably a high efficiency (beyond 90% demonstrated in the continuous regime), and an architecture that is well suited to a very large number of beams (typically >100) on account of this collective positioning, of a possible two-dimensional arrangement, and of the use of a single lens. On the other hand, this technique may not apply as is in the ultra-short pulse regime.
The technical problem to be solved consists in transferring as efficiently as possible the energy of each of the laser pulses to a single pulse by a coherent process, while degrading the beam quality of the final pulse as little as possible with respect to the elementary pulses, while being compatible with a large number of summed pulses, and also sub-picosecond pulse duration.
The proposed solution is based on recombination by superposition using a diffractive optical element DOE to combine the beams. According to the invention, an optical diffractive assembly is placed upstream of this diffractive optical element so as to make it possible, via an appropriate imaging system, to optimize the combining efficiency in the ultra-short pulse regime.